The present invention relates generally to superplastic forming, and particularly to the control and prevention of the formation of cavities in superplastically formed parts by sensing the onset of acoustical emissions caused by cavity formation.
One possible mechanism for the formation of voids or cavities is that, as one grain of a material slides past another grain during a forming process, a void may form along the boundary between the grains, frequently at the intersection of three grains. The void is eliminated if adjacent material diffuses into the void to fill the same.
U.S. Pat. Nos. 4,354,369 and 4,516,419 to respectively Hamilton and Agrawal show methods for reducing and preventing the formation of cavities or voids in metal parts formed by superplastic forming. Hamilton, for example, employs pressure to both sides of a blank workpiece either during the forming or after completion of forming. If applied during forming, the pressure reduces the magnitude of the tensile hydrostatic or mean stress acting on sites of cavity nucleation, thus preventing the formation of voids or at least decreasing the size to which they grow and/or the number of voids. The imposition of pressure to both sides of the blank adds a compressive hydrostatic stress component to the normally generated tensile hydrostatic stress to provide a net hydrostatic component of reduced tension or even a compression. Cavitation is thereby reduced or eliminated because forming is accomplished while the voids or void nucleation sites are subjected to reduced tensile stresses.
Similarly, Agrawal applies fluid pressure on both sides of a blank in a superplastic forming process such that the opposing pressures suppress or eliminate cavitation and thereby reduce chances of rupturing of the blank while it is being formed into a component part. Agrawal notes that the elimination or substantial reduction of cavities is essential to enhance tensile elongation and to ensure desirable properties in the product so formed. As in the above Hamilton patent, Agrawal, in using high pressures on both sides of the blank at the beginning of the forming cycle, provides compressive stress over the entire blank of material to suppress initiation of cavities. In addition, a high pressure maintained on the side of the part opposite the forming surface of the die after completion of forming also provides a similar compressive stress over the entire part to affect closure of cavities that may still remain in the part.
Another publication disclosing the use of back pressure to control cavitation during superplastic forming is a paper entitled "Forming Process Variable Effects on Cavitation in the Superplastic Forming of Commercially Produced 7475 Aluminum" by J. M. Story, J. I. Petit, D. J. Lege and B. L. Hazard, as published in Superplacity in Aerospace-Aluminum, Cranfield, United Kingdom, edited by Roger Pearce and Larry Kelly, 1985. J. M. Story is one of the inventors of the present application. This article, inter alia, provides time-temperature profiles and other data for superplastically forming 7475 aluminum alloy pans. The disclosure of this article is incorporated herein by reference.
Presently, mathematical models are employed to predict the rate of pressurization that is necessary to maintain a constant true effective strain rate to provide maximum superplastic formability and minimum cavitation. Such models are disclosed in U.S. Pat. 4,181,000 and 4,233,831 to Hamilton et al, and in 4,713,953 to Yavari, for example. Models that have proven practical in the industry are limited to plane strain and axisymmetric forming. Nonsymmetrical parts are analyzed on a regional basis, treating local regions as plane strain or axisymmetric.